Resonant magnetoabsorption of millimeter-wave radiation in the quasi-two-dimensional organic metals α-(BEDT-TTF${)}_2$MHg(SCN${)}_4$ (M=K,Tl)

The magnetoabsorption of millimeter-wave radiation by single crystals of the organic metals α-(BEDT-TTF${)}_2$TlHg(SCN${)}_4$ and α-(BEDT-TTF${)}_2$KHg(SCN${)}_4$ has been studied in the frequency range 30–120 GHz. The experiments reveal two dominant contributions to the magnetoabsorption spectra. The first is interpreted as the cyclotron resonance of two-dimensional carriers, and is characterized by broad lines (linewidth/magnetic field ΔB/B∼0.5–1). In addition to a resonance exhibiting a cyclotron mass ${\mathit{m}}_{\mathit{c}}$∼2.8${\mathit{m}}_0$, there are two further lines corresponding to frequency-dependent cyclotron masses in the ranges ${\mathit{m}}_{\mathit{c}}$∼(1–1.5)${\mathit{m}}_0$ and ${\mathit{m}}_{\mathit{c}}$∼(0.5–0.8)${\mathit{m}}_0$. This frequency dependence is believed to result from many-body effects. The second contribution to the magnetoabsorption is formed by a series of narrow lines with ΔB/B∼0.03–0.1 and amplitudes 5–10 times smaller than the features interpreted as cyclotron resonances. These narrow lines are attributed to a superposition of modes due to antiferromagnetic resonance and conduction-electron-spin resonance (ESR). The feature characteristic of antiferromagnetic resonance is the presence of a mode with a frequency that decreases with increasing magnetic field. The magnetoabsorption structure attributed to ESR consists of a relatively broad maximum upon which a sharp dip is superimposed. This behavior is believed to be analogous to the resonant transparency observed in thin metallic films undergoing ESR. © 1996 The American Physical Society.